About the critical mass and maturity:
Mac OS X still is catching up with others: they just released Java 1.4, X11 is still coming, Cocoa is strong but still improving, Carbon has been having bugs worked out etc
If you followed those developments from let's say 2000, you'd see how much Mac OSX changed.
By critical mass I mean both the coverage of applications range and absolutely killer application (good example for now would be a Clicker)